1. Field of the Invention
The present invention relates to an image formation method for forming an image by scanning a beam and exposing photosensitive material and an image formation apparatus, particularly relates to an image formation method and an image formation apparatus through which a high quality image can be formed by forming a contrast exposed image on photosensitive material.
2. Description of the Related Art
For an image formation apparatus such as a digital copying machine and a laser printer for forming an image using a beam, an image formation apparatus in which a beam modulated according to image information is reflected and deflected by a deflecting system, for example a polygon mirror and image information is recorded, scanning the scanned face of photosensitive material and others using such a reflected and deflected beam is generally known.
Recently, in such an image formation apparatus, high-speed image formation at high resolution is enabled by the appearance of a high-speed polygon mirror, high-speed image processing, simultaneous scanning in a plurality of scanning lines and others, and resolution is being enhanced. However, for the contrast of an exposed image contributing to the definition and quality of an image, as the reduction of a beam diameter is limited by the constraint of an optical system, sufficient improvement is not performed. Therefore, it is difficult to provide a high definition character and line drawn image required in the field of printing and desk top publishing.
Generally, in an image formation apparatus for forming an exposed image on photosensitive material by the on-off modulation of a beam, the energy distribution profile of an exposed image is obtained by the following convolution of the intensity distribution profile Bp(x, y) of a beam imaged on photosensitive material and a modulated pulse profile Mp(x, y): EQU Bp*Mp(x, y)=.intg.Bp(.xi.,.eta.).multidot.Mp(x-.xi., y-.eta.)d.xi.d.eta.(1) .
Therefore, unless a beam diameter is reduced when resolution is enhanced, the contrast of an exposed image is decreased and the reproducibility of gradation is deteriorated. For example, when a modulated pulse profile is replaced with "M2(x, y).tbd.Mp(2x, 2y)" if resolution is doubled, the above expression (1) is as follows: ##EQU1## and to obtain the similar contrast, a beam diameter is required to be halved.
In the meantime, according to the examination of an optical system for imaging, the minimum beam diameter co in the propagation of Gaussian beam is obtained by the following expression: EQU .omega..sub.0 =.lambda./(n.multidot..pi..multidot..theta..sub.beam).
In this case, .theta..sub.beam is an angle at which a beam is focused, .lambda. is a wavelength and n is a refractive index, and if the diameter of a beam incident to an f.theta. lens is D and the focal length of an f.theta. lens is f, .theta..sub.beam is expressed by the following expression: EQU .theta..sub.beam =tan.sup.-1 (D/(2.multidot.f)).
Therefore, to reduce a beam diameter, it is required to reduce a wavelength .lambda. or to extend the diameter D of a beam incident to an f.theta. lens, that is, the diameter of a beam incident to a polygon mirror.
In such a background, a conventional image formation apparatus using a semiconductor laser which emits a beam with a short wavelength is proposed and a beam diameter is reduced by obtaining a beam with a shorter wavelength up to 680 nm, compared with a wavelength up to 780 nm of a general semiconductor laser. If an argon laser or a combination of a semiconductor laser and a wavelength sensing element is used as a light source, a beam with even a shorter wavelength can be obtained.
In the meantime, for another conventional image formation apparatus, technique for simultaneously scanning a plurality of beams horizontally and reducing the rotating speed of a polygon mirror is disclosed in, for example Japanese Unexamined Patent Publication Nos. Sho 51-100742 (1976) and Sho 54-38130 (1979). According to these image formation apparatuses, a plurality of beams can be simultaneously scanned horizontally, the rotating speed of a polygon mirror can be reduced and as the radius of the polygon mirror can be extended and the width of each face can be secured sufficiently if the rotating speed of the polygon mirror can be reduced, the diameter of a beam incident to the polygon mirror can be extended.
However, according to a conventional image formation apparatus, in the case of the former, as a reduced wavelength is 680 nm, compared with the wavelength up to 780 nm of a general semiconductor laser, only approximately 12% of improvement can be obtained and a beam diameter cannot be reduced to the extent that the contrast of an exposed image on photosensitive material is enhanced. If an argon laser or a combination of a semiconductor laser and a wavelength sensing element is used as a light source, an apparatus is large-sized, costs are increased and as the sensitivity in a short wavelength area of a general organic photosensitive material in the current electrophotographic process is low, there is a problem that image formation is difficult.
In the case of the latter, as the effect of a polygon mirror upon windage loss torque more greatly depends upon the diameter of the polygon mirror than the rotating speed, the radius of the polygon mirror cannot be extended so much and there is a limit in extending the diameter of a beam incident to the polygon mirror. Therefore, a beam diameter cannot be reduced to the extent that the contrast of an exposed image on photosensitive material is enhanced.